Astronomers recently launched a large-scale search for radio signals from extraterrestrial civilizations on the famous exoplanet K2-18b, which is about 124 light-years away from the Earth. The results did not find suspicious artificial narrow-band radio emissions, but it significantly improved the technical capabilities of the future search for extraterrestrial intelligence (SETI).
K2-18b is located in the constellation Leo, orbiting a red dwarf star in the habitable zone of the star. It is one of the most intensively studied exoplanets in recent years. Observations show that the planet's atmosphere is rich in carbon dioxide and methane, a combination that makes it a popular candidate for a "Hycean planet" - a type of world with a thick, hydrogen-rich atmosphere and possibly a global liquid ocean underneath. This characteristic also makes K2-18b one of the targets monitored by SETI researchers.
In the latest observation, the scientific research team used the Carl Jasper Jansky Very Large Array (VLA) in New Mexico, USA, and the MeerKAT radio telescope in South Africa. Both of these instruments are currently one of the most powerful radio telescopes on earth. This joint monitoring operation is also quite rare in terms of observation arrangements. However, the hardware is only part of the job. In radio astronomy, the software used to sift and filter the data is equally critical, because a large number of signals received by telescopes come from human activities on Earth and must be distinguished from potential celestial signals through complex algorithms.
In this study, VLA used the Commensal Open-Source Multi-Mode Interferometer Cluster system and MeerKAT used the Breakthrough Listen User Supplied Equipment (BLUSE) system, which are currently key tools for screening "suspicious signals" from massive background noise. However, how to set up the screening logic is still the responsibility of the researchers. The paper mentioned that the team imposed five constraints on the data processing to find possible "technosignatures" from millions of signals.
First, the researchers conducted radio frequency interference (RFI) shielding on frequency bands known to be severely affected by terrestrial radio interference, and eliminated all signals falling within these frequency bands; if it is assumed that alien civilizations "just happen to" use these frequencies, humans may need to deploy radio telescopes on the back of the moon to truly hear each other. Secondly, the team used the Doppler effect for screening: signals propagating between planets should show obvious frequency drift due to relative motion, so any signal with almost no Doppler change is regarded as coming from the Earth itself and is directly eliminated.
In terms of signal-to-noise ratio (SNR), the research team made a controversial choice: eliminating signals with an SNR lower than 10 or higher than 100. This strategy helps rule out very weak false alarms and strong instrument artifacts that usually only appear in a single antenna, but it also has the potential to filter out real but relatively weak alien signals altogether. The study also used multi-beam analysis: the telescope formed multiple "beams" in the sky, one of which was pointed directly at K2-18b, and the others pointed elsewhere. If the signal came from the planet, it should only appear in the beam aimed at K2-18b, while terrestrial interference often appeared in multiple beams at the same time.
Another potential method is "transit filtering" - if the signal comes from the planet itself, it should temporarily disappear during the phase when the planet eclipses the star. However, since no "secondary transit" of K2-18b occurred during this observation window, this check was not actually enabled.
After applying the multiple filters described above, the team reports that although millions of potential signals were detected throughout the observation window, none passed all filters and ultimately found no narrowband artificial radio "technical signature" that could be attributed to K2-18b. This result may be slightly disappointing to those who expect to "hear" alien civilizations, but for science, this is exactly the process needed to move forward.
By thoroughly scanning the planetary system and confirming "no detections", the research team was able to set an "upper limit" on the power of the transmitters that may be present in the system. The paper points out that if there is civilization there, its radio transmission power will at least not be significantly stronger than that of ground facilities like the collapsed Arecibo radar in Puerto Rico. In other words, the other party does not use a huge radio "lighthouse" that far exceeds this level to shout loudly to the universe.
The more important result is that this work is a "proof of concept" for the automated filtration system. The millions of signal records generated in the joint observations of VLA and MeerKAT are almost impossible to rely on manual inspection one by one, and the algorithms and software processes demonstrated in this study lay the foundation for future large-scale searches. When larger radio facilities, such as the Square Kilometer Array, come into use, these technologies will help researchers efficiently screen potential signals from extraterrestrial civilizations from the larger flood of data.
The research team pointed out that although K2-18b is still "quiet" at present, humans' ability to monitor the universe is constantly improving. Once intelligent life there or in other worlds begins to "speak," humans will be more capable of capturing these calls from the depths of the universe for the first time. The relevant results have been published on the preprint platform arXiv in the form of a paper titled "Narrow-band technical signature search for the Hycean candidate planet K2-18b using VLA and MeerKAT."